The present invention relates to a blow device for a dryer section of a paper machine by means of which a blowing is produced to support a paper web or equivalent. The device comprises means defining an air space or chamber into which the air to be blown is passed through an associated duct, and nozzle openings communicating with the air chamber and through which the air is blown so as to support the paper web or equivalent.
In recent years, the running speeds of paper machines have become higher and as a result, fluttering of the web and its separation from the support fabric have caused problems which are detrimental to the runnability of the paper machine. These problems have occurred in particular in connection with single-wire draw, in which the web runs from a first row of cylinders to a second row of cylinders on support of a drying wire whereby on the first row of cylinders, the web is situated between the drying wire and the cylinder face, and on the second row of cylinders, the web is situated outside and the drying wire is situated between the cylinder face and the web. The web runs over the draws between the rows of cylinders on support of the drying wire. It is an advantage of this single-wire draw that the web is constantly supported by the drying wire and the web has no open draws, or at least no substantially long open draws, whereby the risk of wrinkles in and breaks of the web is reduced.
It is well known in the art that a think layer of air follows the moving faces so that no relative movement takes place between the air and the moving face, but rather those particles of air that are in contact with the moving face travel at the same speed as the face. In dryer sections of paper machines, these moving layers of air produce various flows and, with a view toward controlling the problems resulting from these flows, in paper and board machines, in the area of single-wire draw, various blow boxes have been used for supporting the web. One blow box of this sort is described in Finnish Patent No. 69,332 corresponding to U.S. Pat. No. 4,628,618.
Circulation air of the paper machine has often been used as the air in these blow boxes, which circulation air may contain various contaminations, such as dust, which may block the nozzle slots in the blow box so that the operation of the blow box deteriorates. For this reason, the blow boxes must be cleaned at regular intervals by various means. Generally, the blow boxes are washed with an abundant amount of water during a regular standstill period of the paper machine, for example, about four times per year. In the prior art, arrangements are known in which the blow boxes are cleaned from outside by means of a washing device which is attached, for example, to a mobile sledge. Arrangements have also been suggested in which the blow boxes are washed from inside, and in this respect reference is made to Finnish Laid-Open Publication No. 91,176 corresponding to W.I.P.O. publication no. WO 94/28241. In spite of the arrangements of cleaning, problems may also have been caused in blow boxes by a situation in which pieces of paper or board, which have been detached, for example, in connection with a web break, have had access to the nozzles in the blow box through the air supply ducts.
In the prior art constructions of blow boxes, the construction has tended to become complicated in an attempt to achieve the desired arrangement of air distribution. Also, quite frequently, the prior art devices constitute heavy constructions.